Lipids: Nonpolar and Polar, Formation and Functions - Prof. Jerry J. Brand, Study notes of Biology

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BIO 311C

Spring 2009

Lecture 15 – Wednesday 25 Feb.

Lipids are biological molecules that (1) are not water soluble (i.e. - they are hydrophobic), and (2) do not fit into any other major category of biological molecules. Nonpolar lipids are typically mostly carbon and hydrogenatoms. They do not contain a high proportion of oxygen ornitrogen atoms. Polar lipids generally contain a high proportion of oxygenand/or nitrogen atoms. At least one end of the molecule ishydrophilic although the hydrpphobic portion keeps it fromdissolving in aqueous solutions. In this course we distinguish several rather similar terms: - polar covalent bond - polar bond (polar bonding) - polar molecule - polar lipid

Review

H O 2 diglyceride + phosphate phosphatidic acid + H

O

acid + alcohol

ester + water

dehydration rx.

phosphatidic acid contains 3 ester bonds, two carboxylic acid ester bonds andone phosphate ester bond.

dehydration reaction

phosphatidic acid is a polar lipid.

H O 2 phosphatidic acid + R polar phospholipid + H

O

dehydration

reaction

phosphodiester bond

Phospholipids are very polar lipids

Abbreviated Structure of Cholesterol Cholesterol is a nonpolar lipid, although the alcohol functional group gives itslight polarity and is very important for its functions.

Isoprene units

Isoprene units are C

5

hydrocarbon portions of molecules with

the chemical structure shown here

structure showing all atoms but not all covalent bonds Very abbreviated structure, where carbon atoms represent ends of straight lines

Illustration of a Polypeptide

amino acids

peptide bonds

Folding and other modifications

Functional Protein Polypeptide (chain)

Structural Formula of an Amino Acid A central carbon atom is attached to four chemical groups. The central carbonis an asymmetric carbon atom unless R is identical to one of the other threechemical groups. central carbon atom Both the amino and the carboxylic acid functional group are ionized exceptat extreme pH values. A single amino acid that isnot chemically attached toanything else is called a"free amino acid". Characteristics of Free Amino Acids

H H 3 N COO C R

The R-group of a few kinds of amino acids carry an amino or acarboxylic acid functional group. These free amino acids carrythree electrical charges. central carbon atom Amino Acids Used to Construct Proteins Since the central carbon atom of an amino acid is asymmetrical, there are 2enantiomers of each kind of amino acid. Cells use only the "L" enantiomer of eachkind of amino acid to construct proteins. Twenty different kinds of amino acids areused for constructing proteins, each witha different "R" group.

Aspartic Acid (shown without its functional groups ionized) How many electric charges would thisfree amino acid have in the cytoplasmicmatrix of a living cell? How many asymmetric carbon atomsdoes this free amino acid contain? Glycine (shown without its functional groups ionized) How many electric charges would thisfree amino acid have in the cytoplasmicmatrix of a living cell? How many asymmetric carbon atomsdoes this free amino acid contain?

H O 2 aa

• aa

dipeptide + H

2

O

dehydration reaction Dipeptide Formation From Two Amino Acids

Atoms of peptide bond

A Peptide Bond Between Two Amino Acids A peptide bond between two amino acids consists ofthe four atoms shown at right. All 6 atoms shaded ingrey (above) lie in the same rigid plane, with thedouble-bonded oxygen projecting in the oppositedirection as the hydrogen atom that is bonded to thenitrogen atom. O C N^ H The carboxylic acid and amino functional groups canno longer ionize after they have become incorporatedinto a peptide bond.

The Amino Acid Sequence of a Polypeptide Chain Modified from textbookFig 5.21, p. 82 Polypeptide chains have twodistinctive ends, an amino-terminal (N-terminal) end anda carboxyl-terminal (C-terminal) end. Polypeptide chains are neverbranched, but are alwaysunbranched chains and arecalled "linear" chains.